While display devices based upon laser light sources have been known for some time, their performance limitations have presented a barrier to many desirable applications. This is especially true for color display applications where the complexity and cost of using readily available, multiple, independent laser devices precludes large acceptance of these devices. Additionally, although lasers are an ideal light source for such applications from an intensity or brightness perspective, the coherence of the laser light generates speckle that is unpleasant to the viewer.
Display devices based on laser light sources depend upon the generation of multiple wavelengths of light in order to produce a color display. The color is created by appropriately mixing such light sources to achieve a high color quality; the measure of the color range is termed the gamut. Three or more different wavelength sources are commonly used to create a high gamut display. These distinct light sources are projected onto a surface from which the content is viewed. Commonly two forms of such projection are used. Front-projection refers to the delivery of the multiple wavelengths of light using an optical system in which the delivery occurs to the front surface of the viewing screen. The screen is observed by the viewer in a reflection mode. Rear-projection refers to the situation in which the light is delivered by an optical system to the rear of the viewing screen. In this instance, the light propagates through the screen and is observed by the viewer in a transmission mode.
In U.S. Patent Publication US 2001/0022566 A1, Okazaki describes the use of three separate lasers to produce the three different wavelengths for a color display. These wavelengths are produced by a variety of inorganic solid state laser devices. These three wavelengths are combined using optical means well known to those versed in the art, and through the use of galvonometer and rotating mirrors, as well as additional optics, an image is produced upon the viewing screen. The color gamut of this display is dictated by the choice of wavelengths available from such solid state laser sources. In turn, these wavelengths are determined by the solid state material properties, including such properties as the solid state alloy composition, dopant type and composition, etc. In this apparatus a small mirror is wobbled to reduce the deleterious effects of speckle for the viewer. Solid state laser sources are preferable for their improved electrical power efficiency, reduced size, and lower cost relative to the more commonly employed gas lasers. Additionally, because this apparatus does not obtain its multiple wavelengths needed for the display by employing nonlinear optical means of wavelength conversion, reduced amplitude noise is claimed for this apparatus.
In U.S. Pat. No. 6,304,237 B1, Karakawa discloses a display apparatus where the three wavelengths for display are produced by a single pulsed laser which is used to produce three different optical wavelength beams by means of nonlinear wavelength conversion. An Nd:YVO4 crystal with laser output at 1064 nm is used to pump a number of different devices to produce three different visible wavelengths suitable for a display system. Wavelengths in the green, red, and blue portions of the spectrum are produced by nonlinear conversion. For example, Second Harmonic Generation (SHG) in an external optical cavity produces the green wavelength at 532 nm. Various other schemes for producing the other visible wavelengths are described including the use of an Optical Parametric Oscillator (OPO) and Sum Frequency Mixing (SFM). Pulsed lasers are used in order to produce the high optical powers required for such nonlinear optical generation schemes, and to meet the brightness requirements for the display. In some instances an etalon is employed in the external optical cavities used to produce the visible radiation. The etalon is used to generate a multi-longitudinal mode output, which results in the reduction of the coherence of the optical beam. In this way, the undesirable effect of speckle is reduced. The increased amplitude noise generally characteristic of pulsed lasers is not mentioned in this document.
In addition to projection systems where the visible laser light is delivered to the viewscreen after free propagation, i.e., propagation through an atmosphere or empty space, there exist display systems using visible laser light in which the light is brought to the viewscreen by waveguide action. In U.S. Pat. No. 5,381,502, Veligdan discloses the use of a planar optical waveguide to deliver the laser light to the viewscreen. In principle, such an apparatus would result in a much thinner display, assuming that the laser light source(s) can be adequately reduced in size.
There are a number of problems with past designs that the current invention overcomes. Inorganic solid state laser devices, although an improvement over gas lasers in terms of cost, reliability, and size, are still costly and relatively large devices, particularly for display applications. This makes the creation of “flat panel” displays based upon these devices difficult to achieve. Clearly it is desirable to reduce the size and complexity of such an apparatus. Furthermore, wavelength selection, in order to realize an optimum color gamut or color range, is limited for inorganic solid state laser sources.